Piezoelectric actuator for media flowing therearound and use of a corresponding piezoelectric actuator

ABSTRACT

A piezoelectric actuator for media flowing around it having a piezo stack which is arranged within a deformable isolating material so as to be in direct contact therewith at least over certain regions. The isolating material is for its part enclosed by a fluidically closed actuator housing, which is formed by a housing shell which is delimited at its one end by a dimensionally stable actuator top and at its other end by a dimensionally stable actuator bottom. The actuator top and bottom are arranged on the active main surfaces of the piezo stack. To increase the service life, the housing shell is produced from a limp and/or elastic material and is disposed at a distance from the piezo stack. The length of the housing shell, measured along the surface line, corresponds at least to the maximum extent of the piezo stack and/or the housing shell can be stretched accordingly.

The application claims the priority of German Patent Document No. 102 3032.1, filed 4 Jul. 2002 and PCT/EP2003/005964, filed 6 Jun. 2003 thedisclosure of which is expressly incorporated by reference herein,respectively.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a piezoelectric actuator for media flowingtherearound, as disclosed for example by the generically determinativeGerman application DE 198 18 068 A1, and uses of the same.

The generically determinative German application DE 198 18 068 A1discloses a piezoelectric actuator for media flowing therearound whichcomprises a piezo stack which is arranged within a deformable isolatingmaterial so as to be in direct contact therewith. The isolating materialis enclosed by an actuator housing. The actuator housing is formed by ahousing shell, which is connected at one end to a dimensionally stableactuator top and at the other end to a dimensionally stable actuatorbottom. The actuator top and the actuator bottom are connected to theactive main surfaces of the piezo stack. The electrical connecting linesof the piezo stack are led through the actuator top. Furthermore, it isconceivable to lead the actuator connections of the piezo stack up tothe end plates (actuator top or actuator bottom) of the actuator and touse one or both end plates as electrical contact surfaces.

Since the electrically insulating isolating material is formed from anelastic plastic, for example silicone, which bears directly against theouter surface of the piezo stack, it must follow the very rapid movementof the piezo stack in use. As a result, there is the risk of cracksforming in the isolating material and the isolating material becomingdetached, so that the medium flowing around can get into the piezo stackin a destructive way.

The object of the invention is to increase the service life of suchactuators for media flowing therearound.

The object is achieved by forming the housing shell from a limp and/orelastic material, in order that the hermetic separating layer is movedaway from the highly active surface of the piezo stack. This reduces theeffect of wear between the piezo stack, comprising a piezoelectricceramic, and the isolating material. This is achieved furthermore bysimple structural operations, so that it is possible to dispense withcomplex sealing arrangements.

An actuator according to the invention permits a free change in lengthof the actuator, or of the piezo stack, with the effect of at leastreducing the probability of the piezo stack being wetted by aparticularly aggressive medium flowing therearound, preferably apressurized fuel.

The housing shell is disposed at a distance from the piezo stack at allpoints. Furthermore, the length of the housing shell, measured along thesurface line, corresponds at least to the maximum extent of the actuatorand/or the housing shell can at least be stretched accordingly, in orderthat the extent of the piezo stack can be accepted by the housing shell.

Since the isolating material is preferably formed largely by anincompressible medium, its dimensional change, for example the formationof a constriction in the extent of the piezo stack, can be taken intoaccount in the dimensioning of the length of the housing shell, inparticular in the direction of the extent of the piezo stack.

Furthermore, an electrically insulating fluid, in particular a liquid,and/or a gel is introduced between the housing shell and the piezostack. The isolating material, in particular a silicone oil, at leastlargely fills the inside volume of the actuator housing. In this way,the inside volume of the actuator housing is at least largely free froma compressible gas.

In the case of the claimed construction, only the fluidic isolatingmaterial has contact with the piezo stack. Therefore, any shearingforces that may occur are small, thereby increasing the service life ofthe actuator. Against this background, it is also an advantage that, ifthe isolating material has good to high thermal conductivity, anyfrictional heat occurring between the piezo stack and the material isremoved.

The same applies to the removal of heat from the piezo stack, for whichreason the thermal conductivity of the isolating material is preferablyequal to or greater than that of the material of the piezo stack.

The same advantage applies to the thermal conductivity of the two endplates (actuator top and actuator bottom), whereby the dissipation ofthe heat occurring in active operation to the medium flowing around isfacilitated and/or improved.

Since the media flowing around the actuator, in particular fuel for theoperation of an internal combustion engine, may well have a chemicallyor in some other way aggressive character, material which is at leastlargely resistant to these expected stresses is expediently chosen asthe material of the housing shell.

In a favorable way, the viscosity of the isolating medium correspondsapproximately to that of the medium flowing around, since the loading ofthe material of the housing shell by the medium flowing therearound isfurther reduced as a result.

In particular when using actuators of which the end plate(s) is/are usedas electrical contacts, the material of the housing shell isappropriately an electrically insulating material.

Depending on the application, for example in hydraulics, it is favorableto form the actuator top with a different cross-sectional area than theactuator bottom. In a further refinement, the actuator top and/or theactuator bottom has a cross-sectional area which is adapted to therespective conditions in use.

A preferred use of actuators according to the invention is in and/or asan injection valve, in particular of an internal combustion engine,preferably in a gasoline or diesel engine. Furthermore, such an actuatormay also be used for a proportional valve and/or for a sonotrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail on the basis of exemplaryembodiments that are represented in the figures, in which:

FIG. 1 shows an exploded representation of an actuator,

FIG. 2 shows an actuator as shown in FIG. 1, arranged in a medium and inthe relaxed state,

FIG. 3 shows the actuator as shown in FIG. 1 in the completely extendedstate,

FIG. 4 shows an actuator with opened actuator top and electrical linesled out from the actuator housing on one side,

FIG. 5 shows an actuator with electrical lines arranged on both endplates,

FIG. 6 shows an actuator with different end plates and elastic materialfor the housing shell and

FIG. 7 shows the actuator as shown in FIG. 6 with an extended piezostack.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, an exploded drawing of an actuator according to the inventionis represented. The actuator has inside it a centrally arranged piezostack 1 comprising a number of layers of piezo sheets of a piezoelectricceramic.

The piezo stack 1 is surrounded by an electrically insulating isolatingmaterial 3, in particular a silicone oil. The isolating material 3 issurrounded on the outside by a housing shell 4, which is sealed withrespect to the isolating material 3.

Arranged preferably equidistant from each other on the active mainsurfaces of the piezo stack 1, there is on the one hand a dimensionallystable actuator top 5 b and on the other hand a dimensionally stableactuator bottom 5 a.

The actuator bottom 5 a and the actuator top 5 b are both connected in asealing manner to the housing shell 4, with respect to the isolatingmaterial 3 and with respect to the medium flowing around. The actuatorbottom 5 a, the actuator top 5 b and the housing shell 4 together forman actuator housing which is sealed at least with respect to theisolating material 3 and the medium flowing around.

Electrical connecting lines 2, which lead to the actuator 5 b, are bothconnected to regions of the actuator top 5 b in such a way that they areelectrically conducting but nevertheless insulating with respect to eachother. One of the connecting lines 2 is respectively connected tounipolar regions of the piezo sheets. They serve for supplying voltageand for controlling the extent of the piezo stack 1. In this way, thecorresponding regions of the actuator top 5 b that are connected tothese connecting lines 2 represent contact surfaces for the electricalcontrol of the actuator.

In the present exemplary embodiment, the housing shell 4 consists of alimp, preferably tear-resistant material. It is disposed at a, notnecessarily constant, distance from the piezo stack 1 at all points.

In FIGS. 2 and 3, the actuator shown in FIG. 1 is represented in a fuelfor internal combustion engines, preferably gasoline or diesel engines,which is flowing around it and under pressure (see arrows). In themoved-together state, the housing shell 4, made of limp material, is ofa form which is irregular and pushed-together or pressed-togetheraxially (along the main direction of extent of the piezo stack 1) andradially (see FIG. 2). When the piezo stack 1 is fully extended (seeFIG. 3), the housing shell 4 is stretched and approximates a straightline.

As can be seen, the minimum height of the housing shell 4, measuredalong the surface line, corresponds at least to the correspondingmaximum extent of the actuator. The minimum height also includecompensation for the deformation of the isolating material 3, as mayoccur at least when the piezo stack extends.

The deformation of the liquid and/or gel-like isolating material 3,preferably a silicone oil, is based on the incompressibility of fluids.Since the volume of an incompressible isolating material 3 remains thesame when the piezo stack 1 extends, it must change its shape. This maytake place for example—as represented in FIG. 7—in a constricted mannerfrom one cylindrical shape to another cylindrical.

It follows that the minimum length of the shell is usually somewhatgreater than the distance between the two end plates 5 when the piezostack 1 is completely extended (see FIGS. 2 and 3). In the case of apurely limp and inelastic material, the compensation should already betaken into account in the dimensioning of the length for the housingshell. In the case of an elastic material, the compensating lengthand/or the displacement of the piezo stack 1 can be applied inparticular just by the elasticity of the material.

The exemplary embodiment according to FIG. 4 is identical in large partsto that shown in the previous figures. The representation is shownhowever without the actuator top 5 b and without isolating fluid. In thecase of this exemplary embodiment, however, the control lines 7 are ledto the outside through the actuator top 5 b—not depicted—andelectrically insulated from one another.

In the exemplary embodiment as shown in FIG. 5, on the other hand, oneconnecting line is connected in an electrically conducting manner to theactuator bottom 5 a and the other to the actuator top 5 b. In particularin this case, the housing shell 4 should be produced from anelectrically insulating material.

Represented in FIGS. 6 and 7 are actuators of which the actuator bottom5 a and actuator top 5 b have a different cross section. At the sametime, the end plates serve as electrical connections for the piezo stack1. The housing shell 4 is produced from a material which is electricallyinsulating and elastic. In the relaxed state (stress equals zero) of thepiezo stack 1, the length of the housing shell is greater than thedistance between the two end plates 5 connected to them in a sealingmanner, so that the housing shell gives a limp impression.

In the fully extended state of the piezo stack 1 (see FIG. 5), thesurface line of the housing shell 4 does not run in a straight linebetween the two end plates 5 but as a constriction, for the reasonsstated above.

With the present elastic material of the housing shell 4, which isadditionally also limply fitted, in the case of this exemplaryembodiment the compensating length is applied by the greater length ofthe shell in comparison with the drawn-together position of the piezostack 1 and also by the elasticity of the material of the housing shell4.

1. A piezoelectric actuator for insertion in a flowing media,comprising: a piezo stack having a top active main surface and a bottomactive main surface; deformable isolating material in direct contactwith at least portions of said piezo stack; actuator housing enclosingsaid isolating material, said housing having a non-rigid housing shelland a dimensionally stable actuator top arranged on the top active mainsurface of the piezo stack and dimensionally stable actuator bottomarranged on said bottom active main surface of the piezo stack;electrical connection lines extending from said piezo stack through saidactuator housing by means of at least one of said dimensionally stableactuator top and actuator bottom wherein said housing shell is disposedat a distance from piezo stack at all pointer, and a length of thehousing shell, when measured along a surface line, corresponds to atleast a maximum extend of the piezo stack or the housing shell is ableto be stretched to the maximum extent of the piezo stack, wherein theisolating material is one of an electrically insulating fluid and gel,wherein an inside volume of the housing is substantially filled withsaid isolating material, and wherein the housing shell and thedimensionally stable actuator bottom and the dimensionally stableactuator top are connected to each other in a sealed manner with respectto the isolating material and the flowing media.
 2. The piezoelectricactuator as claimed in claim 1, wherein the piezo stack comprisespiezoelectric ceramic.
 3. The piezoelectric actuator as claimed in claim1, wherein the actuator top and the actuator bottom are arrangedequidistant from each other.
 4. The piezoelectric actuator as claimed inclaim 1, wherein the material of the housing shell is at least largelyresistant to the media flowing around it.
 5. The piezoelectric actuatoras claimed in claim 1, wherein an inside volume of the actuator housingis substantially gas-free.
 6. The piezoelectric actuator as claimed inclaim 1, wherein viscosity of the isolating medium correspondsapproximately to viscosity of the flowing media.
 7. The piezoelectricactuator as claimed in claim 1, wherein the material of the housingshell is electrically neutral.
 8. The piezoelectric actuator as claimedin claim 1, wherein the actuator top and the actuator bottom have adifferent cross-sectional area.
 9. The piezoelectric actuator as claimedin claim 1, wherein the isolating material is a silicone oil.
 10. Thepiezoelectric actuator as claimed in claim 1, wherein thermalconductivity of the isolating material is equal to or greater thanthermal conductivity of the material of the piezo stack.
 11. Thepiezoelectric actuator as claimed in claim 1, wherein at least one ofthe actuator top and the actuator bottom have a cross-sectional areawhich respectively corresponds to the assigned active surface of thepiezo stack, arranged transversely to the main direction of extent ofthe piezo stack.
 12. The piezoelectric actuator as claimed in claim 1,wherein at least one of the actuator top and the actuator bottom have atleast two terminating regions, electrically insulated from each other,for the electrical connecting lines of the piezo stack.
 13. The use of apiezoelectric actuator as claimed in claim 1 for an injection valve ofan internal combustion engine.
 14. The use of a piezoelectric actuatoras claimed in claim 1 for a proportional valve.
 15. The use of apiezoelectric actuator as claimed in claim 1 for a sonotrode.
 16. Thepiezoelectric actuator as claimed in claim 5, wherein the media is fuel.17. The piezoelectric actuator as claimed in claim 13, wherein theinternal combustion engine is one of a gasoline and diesel engine. 18.The piezoelectric activator as claimed in claim 1, wherein the materialof the housing shell is at least largely resistant to the media flowingthere around, an inside volume of the actuator housing is substantiallygas-free, viscosity of the isolating medium corresponds approximately tothe flowing media, the housing shell material is electrically neutral,and thermal conductivity of the isolating material is at least equal tothat of the piezo stack material.
 19. The piezoelectric actuator asclaimed in claim 1, wherein the non-rigid shell is fabricated from alimp or elastic material.